The use of dry magnetic methods in the cleaning of coal is of interest because of the potential for efficient separation of pyritic sulfur by a safe, environmentally acceptable and inexpensive dry process. The scientific basis for the method is unquestioned: the carbonaceous structure of the coal is diamagnetic and the principal sulfur-bearing minerals, iron pyrite and iron sulfate, are paramagnetic. Additionally, many ash-bearing "non-magnetic" minerals, such as quartz and shale, can also be separated from coal by magnetic methods because they can be made weakly paramagnetic by small amounts of iron impurity naturally associated with these minerals.
Both wet and dry magnetic coal cleaning methods have been investigated over the past twenty years. In spite of this effort, however, magnetic separation methods have not been applied to commercial cleaning of coal because (1) there has been a lack of technical information on the distribution of magnetic material in American coals, and (2) it has not previously been economically feasible to scale up conventional electromagnet technology for application to coal processing. Recent developments in the areas of coal characterization and high field magnet design have made favorable changes in both of these areas.
Presently there are plans both in private industry and in government to build and man stations in space and/or on the earth's moon. Such stations would require an oxygen supply for all inhabitants, both plant and animal. It would be useful to utilize oxygen-containing minerals and ores on the moon for the production of such oxygen.
Additionally, it would be desirable to utilize minerals on the earth's moon for the production of metals, such as iron, calcium, silicon, aluminum, etc., which could be used in situ, or in connection with building a space station or back on earth. Because of the moon's feeble gravitational pull, roughly one sixth that of earth's, it may be far less cumbersome to transport raw building materials produced on the moon to a space station than to transport those same raw materials from earth. Of course, such advantages are further magnified when the materials are used for building purposes on the moon itself.
The lunar soil is known to contain small amounts of the odd isotope of helium, Helium-3, which could be used as a clean burning fuel with deuterium in fusion reactions for generation of electricity on earth or for generation of propulsion power in space. This is of profound significance for the future of mankind because there is enough of this material in the lunar soil to supply the electrical needs of the U.S. for centuries to come if it can be recovered. Present schemes call for use of an inefficient thermal devolatilization process for treating the entire lunar soil [I. N. Sviatoslavsky and M. Jacobs, "Mobile Helium-3 Mining and Extraction System and its Benefits toward Lunar Base Self-Sufficiency," appearing in Engineering, Construction, and Operations in Space, Proceedings of Space 88, ed. by Stewart W. Johnson and J. P. Wetzel, published by the American Society of Civil Engineers, 345 East 47th Street, New York, N.Y. 10017-2398, p. 310 (1988)]. The Helium-3 is known to be concentrated in the mineral ilmenite (FeTiO3) which is found in abundance in lunar mare soils. Concentration of the ilmenite for feedstock to the devolatilization process could greatly reduce the destruction of the lunar surface while significantly improving the technical and economic feasibility of the recovery process. Presently, there are no known processes for concentration of the ilmenite in lunar soils.
On the earth's moon there are several types of mineral matter and ores which could function as feed stocks for processes that would produce oxygen, metals such as iron and silicon, and nuclear fusion fuel such as Helium-3. However, there is presently no commercially feasible method of beneficiating such materials to concentrate the magnetic elements and compounds which would make separation of these elements and materials possible.
Magnetic methods are preferred in the beneficiation of extraterrestrial material because of the unique nature of the lunar regolith and because dry processing is desired. There is no water on the surface of the moon, hence the need for dry soil processing methods. Further, there is no atmosphere on the surface of the moon and virtually no free oxygen is present. Because of this, one does not observe the 3+ oxidation states of ferromagnetic elements such as iron, Fe3+. This, plus the unique presence of solar wind implanted hydrogen, have created unusual components in the lunar soils. The lunar soil has been finely pulverized by meteorite impact throughout millions of years. The impacts release heat and create glassy components and irregular shaped agglutinates containing elemental iron. The agglutinate fractions and "native iron" inclusions are unique to the lunar soil. The agglutinates are a potential source of reduced iron.
At present, there is no single source of information quantifying the distribution of magnetic materials in either terrestrial or extraterrestrial materials. Because of this, researchers and engineers usually plan for some form of testing using available technology in their efforts to determine the feasibility of magnetic beneficiation for their application. This approach yields results which are specific to the beneficiation apparatus at best and yields no analytical basis for extrapolating the test results.
This empirical approach is acceptable in conventional applications where a variety of commercial separators can be tested and where a sufficient supply of test material is available. The method is inadequate, however, in cases where innovative separations technology may be necessary and where the supply of test materials is severely limited, such as lunar soil samples. Most magnetic separators are intended for specific applications and the empirical design procedures employed by the manufacturer cannot be extended beyond the present usage. Indeed, most vendors simply do not know enough about magnetic materials or magnetic separator design to be able to extrapolate to new applications, such as those involving extraterrestrial matter.
At any rate, this empirical approach cannot be used in projecting technology needs for processing lunar soils because these materials are not available in sufficient quantity for this testing and because no lunar simulant suitable for magnetic purposes exists. The agglutinate fraction, which is important to magnetic beneficiation of lunar soils, is unique to the moon because of the presence of the hydrogen reduced iron.